This application is the U.S. national phase of international application PCT/GB01/03507, filed in English on Aug. 3, 2001 which designated the U.S. PCT/GB01/03507 claims priority to GB Application No. 0019825.9. The entire contents of these applications are incorporated herein by reference.
This invention relates to a method and apparatus for signal processing. More particularly, but not exclusively it relates to a method and apparatus for digital signal processing where the signal is, typically, a radar signal.
Current maritime and aviation radar systems operate on the principal of outputting a train of radio frequency pulses in a given field of view, typically 8-16 pulses with an angular spread of 3-4xc2x0, and receiving the beam returned from a target, typically a ship or an aircraft, waves or the littoral environment. The delay of a returned pulse from the time of transmission is a measure of the distance of the object from the transmitting ship.
Doppler radar is a form of radar where an output signal is modulated onto a higher xe2x80x98carrierxe2x80x99 frequency local oscillator (LO) signal, the output signal frequency typically being around 5 MHz and the LO signal frequency around 3 GHz. The returned signal is demodulated by decoupling the LO frequency therefrom.
The amplitude and phase angle rotation rate of the demodulated reflected signal at a given distance, range cell, are respectively indicative of the radar cross-section of the target and also the relative radial velocity of the target with respect to the transmitting ship. For example, if an object is stationary all of the returned pulses will have the same amplitude and phase angle in the range cell containing the object and subtraction of successive returned pulses will return a near zero value for this range cell. The returns from these stationary objects are known as clutter and are particularly prevalent in littoral regions. A typical system has a moving target indicator cancellation figure, i.e. the noise level after two pulses are subtracted from each other, of xe2x88x9260 dB.
However, if the object is moving with a radial component of velocity relative to the transmitter, the phase of the reflected pulses (emitted at different times) within a range cell will vary, the rate of change of the phase between pulses being equal to the Doppler frequency of the object. Thus, the distance, and relative velocity of the object can be ascertained.
However, the LO does not produce an idealised, pure, single frequency. The LO will typically produce a generally symmetric distribution of frequencies about the central frequency which decrease monotonically away from the central frequency (for example, in the case of a local oscillator with a xe2x88x9220 dB point at 1 kHz, a target with 1 kHz Doppler shift relative to stationary clutter must have a reflected intensity (radar cross-section) greater than 1% of the centre peak, clutter, intensity in order to be detected). The spread of the frequency distribution is dependent upon the type and quality of LO used.
The LO also has instabilities associated with it which introduce phase noise into the transmitted pulses, a typical oscillator has a correlation time of approximately 50s. There is a drift in the LO frequency between the transmission and reception of a pulse which can give the impression that clutter is moving.
Where the LO phase drift at successive times is cumulative rather than simply random, it will be nearly constant for a few successive delays (i.e. through successive range bins) after a given transmitted pulse. The phase drift at the same delay after the next pulse will be different, but again it will vary very little over a few successive delays.
The clutter returns can be very large, for example a 3-4xc2x0 width beam is 1 km wide 20 km from the transmitter and therefore xe2x80x9cseesxe2x80x9d a large amount of clutter. Target returns may be as low as 10xe2x88x928 of the clutter returns.
Maritime radar require significant power outputs, typically in the kW to MW ranges, in order to obtain the desired detection range. This necessitates the amplification of the modulated signal. The amplifiers used are, for example travelling wave tubes (TWT), not very efficient at maintaining the signal frequency unchanged during amplification and this introduces a yet further phase drift between the LO output and the returned signal.
Use of very long coherent dwells with many pulses can provide a reduction in the processed phase noise, but this is not a practical option for an multifunctional radar (MFR) operating in surveillance, due to time budget limitations. Phase noise is therefore expected to be a key limitation to target detection in clutter.
Another important feature of some radars (e.g. airbourne, ground vehicle, or maritime) is that they are frequency agile. The ability to switch between transmitted frequencies can be important, for example where there are many ships in a flotilla and interference between radar transceivers is clearly undesirable. Frequency agility is also important for other reasons such as non-cooperative target recognition (NCTR), multipath detection arrangements and target reflection cross-section detectability (RCS).
The provision of frequency agility does however introduce phase noise into the system as it requires a versatile LO which can not be optimised over a broad range of frequencies as for good radar sensitivity virtually all circuits must be synchronised to the LO). Thus, the independence of the detected signal from the LO frequency can be established.
Mechanical vibrations introduce phase noise into the detected signal as they result in relative motion between the transmitter and the receiver. The cost of reducing phase noise by mechanical, or electronic hardware, means is very large and increases the price of low phase noise radar systems significantly.
It is an aim of some embodiments of the present invention to provide a method of signal processing that reduces the phase noise present in processed radar signals.
It is a further aim of the present invention to provide a method of signal processing that ameliorates at least one of the above problems.
It is still further aim of the present invention to provide signal processing means that reduce the phase noise present in processed radar signals.
It is a yet further aim of the present invention to provide signal processing means that ameliorate at least one of the above problems.
U.S. Pat. No. 4,137,533 discloses a method of discriminating a target radar signal from background clutter.
It will be appreciated in principle, and scope of protection sought, that any reference made herein to xe2x80x98radarxe2x80x99 encompasses electromagnetic radiation of any frequency and that any reference made herein to geographically distinct transmitters and receivers does not necessarily relate to a large separation, e.g. km, it can relate to only a few metres.
It will be appreciated that any reference herein to phase drift or phase noise relates to any variation in phase due to instabilities, electronic or physical, within the radar system.
According to a first aspect of the present invention there is provided a method of discriminating a time variable target radar signal from a background including the steps of:
I) acquiring range variable returns from a series of radar pulses having variable amplitude and phase;
II) sampling the returns to produce a set of ranged signals attributable to a set of range cells, the ranged signals having variable amplitude and phase for each range cell;
characterised by the steps of:
III) obtaining an estimate of the variation in phase drift between the transmission and reception of the ranged signals for each range cell;
IV) producing a smooth function representative of the variation in phase of the ranged signals between nearby (e.g. successive) range cells in each of the returns;
V) modifying the acquired range variable returns with respect to the function representative of the variation in phase between nearby range cells in order to obtain a corrected value for the amplitude and phase of each return signal;
VI) operating on the corrected set of range variable returns so as to identify an object having a relative velocity with respect to a receiver in a range cell.
Preferably the variation in phase between successive range cells in a specified return is obtained via a method including the steps of:
a) producing a smooth function representative of the amplitude and phase of the return signals in each specific range cell in successive returns;
b) modifying the amplitude and phase in each range cell to the corresponding amplitude value of the function for every range cell and pulse;
c) obtaining the phase drift of the (LO) from the modified signal.
The amplitude and phase of the signal may be represented in a complex form having an amplitude and an argument.
Preferably step IV) of the invention is carried out by successively averaging the phase variation in range cells adjacent successive range cells. Alternatively step IV) of the invention may be carried out by fitting the inter range cell phase variation to be a polynomial function, preferably a low order polynomial (e.g. zero, first, second, or third order).
Advantageously in step VI) of the first aspect of the present invention the transformation of the smooth function from phase space to frequency space is carried out by application of a Fourier Transforms (FT) to the smooth function. More advantageously the FT is a Fast Fourier Transform (FFT).
The amplitude of a given range cell may be ignored in step a) if it is below a threshold value. This is in order to overcome amplitude and phase variations being dominated by thermal noise. The smooth function in step (a) may be a low order polynomial.
In step c) the phasedrift of the LO may be obtained by measuring the argument of the modified signal.
Preferably the radar is a maritime radar. The radar may be multistatic radar, i.e. a transmitter and a receiver may not be in the same place. The transmitter may be a transmitter of opportunity. The method may obviate the necessity for a local oscillator at the receiver. The transmitter, a satellite or alternatively it can be a television transmitter. (the radar signal may be bounced off a satellite, covering the satellite to be an effective transmitter).
The variation in phase will be termed phase noise. Preferably the phase noise is due, at least in part, to clutter. Alternatively, or additionally, the phase noise may be due, at least in part, to effects due to the bandwidth of a local oscillator and/or drift in the local oscillator frequency. The local oscillator may be a voltage controlled oscillator. Alternatively, it may be a phase locked oscillator or a synthesiser, for example a direct digital synthesiser (DDS). As a further alternative, or addition, the phase noise may be due, at least in part, to a drift in frequency introduced by an amplifier or may be due at least in part to relative motion between the transmitter and the receiver, for example mechanical vibration.
The pulses may be compressed. The pulses may be digitally compressed. The pulses may be compressed by xe2x80x98chirpingxe2x80x99. Alternatively, the radar signal may be a frequency modulated continuous wave (FMCW).
The pulses may have a carrier frequency in the GHz range, for example a carrier frequency of 3 GHz, 5 GHz, 10 GHz, 20 GHz, or 60-70 GHz. The carrier may be generated by a frequency agile local oscillator. The carrier may be a signal with a frequency in the MHz range modulated onto it, for example a 5 MHz, 10 MHz or 15 MHz signal.
The relative velocity between the receiver and object may be represented by a frequency. This frequency may be a Doppler Frequency.
Preferably the target signal to noise+clutter ratio is about xe2x88x9270 dB. More preferably it is about xe2x88x9280 dB, or over about xe2x88x9290 dB. The target signal to noise+clutter relates to sub-clutter visibility of targets.
According to a second aspect of the present invention there is provided apparatus for discriminating a target time variable radar signal from a background, the apparatus including, a receiver and signal processing means, a transmitter transmits a series of pulses, the receiver being adapted to receive at least a portion of the pulses returned from a target and clutter, the receiver passing the returned portion of the pulse to the signal processing means, the signal processing means being adapted to reduce the phase noise present on the signal and thereby enhance the visibility of the target, in use.
Preferably the signal processing means is a digital signal processing means. The digital signal processing means may be in the form of hardware or alternatively it may be in the form of software stored on a computer readable medium. Preferably the transmitter is located near to the receiver.
According a further aspect of the present invention there is provided a method of signal processing including the steps of:
I) acquiring a plurality of datasets having first and second characteristic variables;
II) sampling each of the datasets to provide a plurality of series of data points having first and second characteristic variables;
III) fitting a smooth polynomial function to a variation of the first characteristic variable between successive data points;
IV) transforming the smooth polynomial function in the first characteristic variable to a third characteristic variable of the dataset.
Preferably the variation of the first characteristic variable is obtained by fitting a smooth polynomial function to the second characteristic variable at individual data points, between successive datasets and normalising each the individual data points in each dataset with respect to the smooth polynomial function.
Preferably the signal to be processed is a radar signal. The datasets may be return signals from a series of output radar pulses. The first characteristic variable may be the phase of the signal and the second characteristic variable may be the amplitude of the signal. The data points may be indicative of distance from a receiving station, these data points can be termed range cells.
The third characteristic variable may be a frequency. This frequency may be characteristic of a property of an object, such as its relative radial velocity, for example a Doppler frequency.
Preferably the amplitude in a given dataset is ignored if it is below a threshold value.
A method of reducing phase noise in radar comprising using radar returns (e.g. from clutter) in range bins that are nearby to each other to form an estimate of the phase draft between the local oscillator of the receiver and the transmitter pulse; and using that estimate of the phase drift to assist in increasing the differentiation between clutter returns and target returns.
Preferably the returns from adjacent range bins are used to establish an estimated phase drift.
Preferably the received, detected, radar returns are processed using the estimated phase drift and the processed values high pass filtered to pass only returns which have a greater phase change than the filter threshold value.
Preferably the phase drift for returns in a given range bin used in the modelling and/or used as the putative target return is not a single measured phase drift for that return, but is instead a value that has been modified by the art of an averaged or modelled phase drift derived from a plurality of pulses in that range bins.
The averaging/modelling of phase drift, within a range bin but across different radar pulse returns, gives a more accurate average phase drift that is related to the clutter return in that range bin.
A method of compensating for phase noise in a radar comprising detecting a plurality of radar pulse returns (1 to x) in each of a plurality of range cells (1 to n); each pulse return having a phase xcex8 (xcex8, . . . xcex8x)n; establishing a relative phase difference (xcex4xcex81 xcex4xcex82 . . . xcex4xcex8x) for each of the plurality of radar returns (1 to x) in each individual range cell (1 to n), the phase difference (xcex4xcex81 xcex4xcex82 . . . xcex4xcex8x) being relative to a modelled value (xcex8mod1, xcex8mod 2, . . . "THgr"mod x) applicable for each particular radar return (1 to x) in the range cell; establishing for each range cell (1 to n) an averaged phase difference (xcex4xcex81 xcex4xcex82 . . . xcex4xcex8n) for the radar returns in that range cell, averaged over the pulses in that range cell; and evaluating the averaged phase differences for the range cells (xcex4xcex81 xcex4xcex82 . . . xcex4xcex8n) to establish one or more anomalous xcex4xcex81 to n, said anomalous xcex4xcex81 to n being indicative of a moving target.
Working within a given range cell there may be significant variations in the LO Frequency between successive pulses due to the large time delay therebetween. xcex4xcex8 due to L.O. phase noise will vary only slowly between successive range cells for a given pulse. Thus for xcex4xcex8pulse number, range cell number the variation from xcex4xcex81,1 to xcex4xcex81,2 is less than the variation from xcex4xcex81,1 to xcex4xcex82,1. Thus it is possible to model xcex8mod 1 . . . xcex8mod x using a simple model (e.g. straight-line fit, polynomial regression fit etc). This allows us to obtain xcex4xcex81xe2x88x92x for each range cell. If there is a significant change in xcex4xcex8 between adjacent range cells this is indicative of a moving target. It will be appreciated that the variation in xcex4xcex81xe2x88x92x may contain an element due to the slow-speed movement of clutter.
Preferably detected phase difference signals (xcex4xcex82 . . . xcex4xcex8n) from a range cell n are divided by a modelled phase noise error signal for each pulse (xcex4xcex81 modelled xcex4xcex81 modelled . . . xcex4xcex8z modelled). Preferably the modelled phase noise error signals (xcex4xcex81modelled xcex4xcex81 modelled . . . xcex4xcex8z modelled) are derived by predicting their value for signal x using the measured xcex4xcex8 from one or more adjacent range bins, preferably immediately adjacent range bins. For example the modelled phase noise error signal (xcex4xcex81modelled xcex4xcex81 modelled . . . xcex4xcex8z modelled) for a range bin n are derived by averaging to xcex4xcex8bin nxe2x88x921 and xcex4xcex8z n+1. If the bin nxe2x88x921 or bin n+1 contains a target averaging their xcex4xcex8 to provide a contribution to xcex4xcex8bin n, pulse x will not be helpful. Instead we may xe2x80x9ccoastxe2x80x9d over the anomalous signal, possible by assuming that xcex4xcex8bin n is the source as xcex4xcex8bin nxe2x88x921, or that it follows any detectable/predictable trend for xcex4xcex8bin near n.
The invention, in at least one embodiment, resides at least in part in the realisation that clutter signal (slow moving) is smoothly varying from pulse to pulsexe2x80x94and so we can average/smooth the signals of a group of pulses (near in time to each other) and use the modelled/smoothed values as a base line from which to evaluate the change in phase of the different detected pulses in that group of pulses. Any large variation in change in phasexe2x80x94away from the smoothly varying predicted valuexe2x80x94is likely to be indicative of a target.
Also at least part of an embodiment of the invention resides in the realisation that the phase noise from a local oscillator or amplifier is constant for a given pulse and since the phase noise varies slowly the phase change in the phase noise varies smoothly from range cell to range cell for a give pulse. Thus we can take, for a given pulse, a predicted, modelled, averaged or otherwise smoothed or best-fitted, value for the phase noise for that given pulse based on the phase noise from different nearby range cells.
Thus for a given pulse in a given range bin we can smooth phase noise by averaging/best-fitting detected phase difference to a line derived from the phase noise detected for that pulse in adjacent range cells. A close fit of the measured change of pulse predicted by smoothing (for each pulse separately) over nearby range cells is indicative that the detected phase noise for that pulse has no target contribution: wide derivation from the predicted value is indicative that there is a target in that range bin.
This invention describes a method of tracking the phase drift from sample to sample (range bin to range bin) and then recalculating the returns applying compensation to allow for this tracked, phase drift.
The PNR technique exploits clutter returns of opportunity to calculate the phase drifts between transmitter and receiver and then modify the outputs to account for these drifts. The method relies on the relative coherence of the phase drifts from range-cell to range-cell compared with those from pulse-to-pulse.
The technique uses radar returns from clutter in successive range bins to form an estimate of the phase drift between the local oscillator and the transmitted pulse. The phase drifts are typically highly correlated from one range cell to the next allowing a high pass filter to be used to reduce the phase noise.
The principle underlying the suggested, signal processing method is to use the near-constancy through successive range bins to estimate the actual phase shift and hence to correct for it. Since the essence of the approach is to estimate the phase deviation from the signal returns and apply correction, it can be classed as adaptive processing.
The predictive (non-white) part of local oscillator noise can be cancelled by adaptively processing the clutter returns in a succession of range bins.